The present invention relates to field effect electron emitters and an improved method for forming field-effect electron emitters, a form of cold cathode electron emitters for a cathodo-/electro-luminescent flat panel display.
A cathodoluminescent field-effect flat panel display consists of a two-dimensional cold cathode with a matrix addressing scheme constructed above the cathode whereby electrons may be allowed to selectively flow with an intensity determined by the matrix addressing scheme to a distally disposed, phosphor-bearing, anodically-biased plate. The display is termed xe2x80x9cfield effectxe2x80x9d because the source of electrons arises from an array of needle-like emitters (from one to several hundred per picture element, or pixel). A positive potential is applied between the first electrode in the addressing scheme and the needle-like emitters or cold cathode structures. The mechanical shape of the needle-like structure produces a compression of equipotential lines and, therefore, an enhancement of the electric field at the tip of the structure by many orders of magnitude. This enhancement is sufficient for the emission of electrons. The material itself, especially its work function, is important to predict the effect. Electrons experience an acceleration through an electric field that is maintained between the matrix addressing scheme and the anodic plate of the device. The anode consists of an array of cathodoluminescent phosphors. Generally, these phosphors are deposited within a black matrix and the entire layer is covered with an aluminum film, very similar to the construction of the face plate of a color cathode ray tube (CRT) however no shadow mask is involved.
As is usual in flat panel displays, the drive electronics present a register of signals, one signal for each picture element in a scan line, to vertically disposed matrix lines. Each signal is modulated either in amplitude or duration or both to effect the luminance level desired. For full color, triplexes of phosphors emitting red, green and blue are addressed. Once the scan line is serviced, the data is replaced by the data appropriate to the second scan line. This line is selected and the data applied. This operation continues until all scan lines have been serviced. To avoid the perception of flickering light, the time allowed to service all scan lines is in the order of 10 to 15-milliseconds. If the phosphorescence is short, each pixel must be restimulated in a shorter time. An alternative is to fabricate a storage means at every pixel. Generally, this is done by fabricating a thin film field effect transistor at every pixel. This electronic storage thereby compensates for the lack of phosphor or other electro-optical persistence. As yet, this alternative is employed commercially only in liquid crystal displays called xe2x80x9cTFT-LCDxe2x80x9ds.
Cathodoluminescent phosphors generally demonstrate both direct luminescence when light is emitted while the phosphor is being driven, and phosphorescence when the metastable phosphor returns to ground state after stimulation. Photons are emitted as electrons fall back into lower energy orbitals. The timing of these effects and human eye physiology determines the maximum time allowed to service all scan lines. The reciprocal of this time is the refresh rate, the number usually reported in display literature. At some point, long phosphorescence times become undesirable because images, intended to change dynamically, smear.
Additional important considerations arise from considering the useful service life of the device. Phosphors exhibit coulomb-aging. Each phosphor material is characterized by the total number or coulombs that the material can accept to lose half its emission efficiency. From this point of view, using higher voltage phosphors at low current for a given energy is better than using lower voltage, higher current phosphors. Ideally, a field effect device (FED) uses CRT phosphors that benefit from 60 years of development. These phosphors have useful lives of up to 20,000 hours of operation. Unfortunately, CRT phosphors operate at high voltage (greater than 6,000-volts). This creates an engineering problem. The anodic plate must be distally disposed farther from the cathode/grid structure. Holding the anodic plate in appropriate position requires intervention to focus electron bundles and spacer members with challenging aspect ratios. The spacer members must be tens of millimeters high but roughly 0.025 millimeters in cross section.
Seminal work in cold cathodes was reported by Spindt et al., in xe2x80x9cPhysical Properties of Thin-film Field Emission Cathodes with Molybdenum Cones,xe2x80x9d Journal of Applied Physics, Volume 47, No. 12, December 1976, pages 5248-5263. Spindt et al., discuss field emission cathode structures in U.S. Pat. Nos. 3,665,241, 3,755,704, and 3,812,559. The flat panel display industry refers to the process that produces arrays of molybdenum cones as described by Spindt et al., as the Spindt process. This process demonstrates the availability of currents in the range of 50-150 microamperes per cone. The service life of emitters using the Spindt process can be limited by ion polishing of the cones. The electric field at a sharp tip is inversely proportional to the radius of the tip. Extremely sharp tips with a radius of tens of nanometers may be dulled to a radius in the hundreds of nanometers by suffering ion impact, or sputtering. It is important to maintain a high quality vacuum in an FED to minimize this effect. Molybdenum has a work function on the order of 4.5 to 5 eV, and therefore, offers no enhancement of the emission effect due to work function.
An example of a method of enhancing the efficiency of a needle-like cold cathode array is given in U.S. Pat. No. 5,908,699 (""699) entitled xe2x80x9cCold cathode electron emitter and display structurexe2x80x9d. The ""699 reference discloses the use of nano-crystalline carbon to create a robust needle-like tip. However, carbon has a relatively high work function (about 5 eV) and therefore, such displays require relatively high potential differences between the cathode needles and the addressing matrix electrode. With the introduction of cesium as a cathode material in ""699, the effective work function is reduced to the order of 1.05 to 1.3 eV, dramatically reducing the operating voltages (e.g., on the order of 4 or 5 to 1). However, cesium is a difficult material. Cesium tends to act as a scavenger in vacuum devices. In fact, cesium is often introduced into vacuum devices as a getter. Cesium is difficult to handle in manufacturing since it is unstable in air. While a low work function material seems highly desirable, cesium may not maintain its properties over the required lifetime of the cathode.
Certain phosphors, for example, ZnS:Mn, emit light when stimulated by an electric field. This phenomenon is called electroluminescence and the flat panel displays predicated on this phenomenon are called electroluminescent displays (EL). Commercial devices are made in two basic ways: (1) powder EL generally uses a thick phosphor layer in a direct current grid work and (2) thin film EL generally uses a layered structure that includes a thin film of EL phosphor and at least one transparent insulator, for example yttrium oxide (Y2O3), in a conductive cross grid. Both structures, powder and thin film EL, are simple, and the display is fabricated on a single insulating plate. Since the luminance arises only in the driven phosphor, one of the conductive electrodes must be substantially transparent. Usually, indium tin oxide (ITO) is used for the transparent electrode. A typical vertical structure is ITO, Y2O3, ZnS:Mn, Y2O3, and Al. Usually, ITO forms vertical lines and aluminum forms horizontal lines. If Al is deposited first, a transparent cover plate is needed, and the initial insulated plate need not be transparent. If ITO is deposited first, the substrate must be transparent. This type of display is commonly referred to as xe2x80x9cacTFELxe2x80x9d, meaning a thin film electroluminescent display that uses an alternating current (ac) drive. The phosphor layer is usually about 0.5 to 2 xcexcm thick. The other layers are in the range of 200 nm thick. The layers are fabricated using conventional thin film deposition techniques such as e-beam deposition or sputtering and, more recently, by atomic layer epitaxy (ALE).
The acTFEL device is somewhat more efficient than the dc powder device and enjoys a greater state of development. Electrically, an acTFEL device exhibits a threshold voltage in the neighborhood of 150 to 175 volts rms. Below this voltage, this device does not emit light. In perhaps a voltage difference of 20-volts rms above threshold, the luminance of a cell may rise from 1 cd/m2 to 800 cd/m2. The highest luminance is usually quite uniform. This makes the technology an easy candidate for an on/off display for displaying graphics and alphanumerics. The luminance also depends on the frequency of the ac drive. For low frequencies, the luminance is linearly related to frequency. At a frequency that depends on structural nuances, the luminance falls off above a peak. The voltage impressed across the phosphor is given by:       V    phos    =                    ε        phos            xc3x97              t        diel            xc3x97              V        applied                                      ε          phos                xc3x97                  t          diel                    +                        ε          diel                xc3x97                  t          phos                    
Where:
Vphos is the voltage across the phosphor,
∈phos is the dielectric constant of the phosphor (about 9.6 for ZnS),
tdiel is the thickness of the dielectric,
Vapplied is the applied voltage,
∈diel is the dielectric constant of the dielectric, e.g., yittria about 4,
tphos is the thickness of the phosphor layer.
For a lower voltage device, these quantities should be adjusted to have a coefficient that approaches one. This must be achieved with a sufficient guard band for the dielectric strength of each layer.
The problems with EL have historically been: 1) xe2x80x9cremanencexe2x80x9d in which an old image is still visible when the intent is to replace the old image with a new image; 2) the lack of a convincing blue-emitting phosphor; 3) the inability of the technology to produce gray scales smoothly; and, 4) poor luminance. Ideally, one would like off, level 1 and then 15 log steps above level 1. Historically, EL has not been able to hold level 1, at least in conventionally scanned panels. Level 1 is too big a step (for example 20% of maximum luminance) and is unstable in both time and physically across the panel. The luminance problem comes from multiplexing. A 768 scan line panel cell emits light only {fraction (1/768)}th of the time. Even so, approximately satisfactory monochrome ZnS:Mn panels show 20 to 30 cd/m2 at such multiplex ratios and convenient refresh rates. International standards (e.g., ISO 9241, part 3, ISO 13406, part 2 and the European Union counterparts CEN 29241, part 3 and so forth) call for a minimum of 35 cd/m2, a realistic minimum is 100 cd/m2. The luminance level is also exacerbated by specular reflectance of the aluminum electrodes. In an ordinary room, one must account for the loss of contrast due to reflected specular sources (see ISO 9241, part 7 for a measurement method and criteria). Often, this problem results in the use of a circular polarizer between the viewer and the display. This reduces the luminance by a factor of more than 2.
Remanence is thought to be caused by charge trapping in the relatively poor quality dielectrics that are realized by e-beam and like thin film deposition techniques. The quality of EL dielectrics is poor compared with insulator systems used in the semiconductor industry that are grown with well-controlled processes using exotic tools. Semiconductor industry insulators are essentially free from trapping centers and may be used with a unipolar bias for 100,000 hours or more. The remanence problem was greatly improved by using drive schemes that seem to guarantee that no average voltage exists across the dielectric-phosphor laminate. However, such remedy was not completely successful.
Multicolor displays are now mandatory in most commercial markets, albeit not required to operate most commercial applications. Blue-emitting phosphors have been invented, but have relatively poor efficiency. A reasonable solution to using these phosphors would be the use of local pixel content storage. The obvious way to achieve this is using thin-film-transistors (TFT-TFEL) at every pixel. Reliable TFTs in the voltage range are still not available.
The precision and stability of the threshold level have been addressed by combining amplitude and pulse-width modulation on the pixel data lines. Threshold stability problems are understood by reviewing some typical numbers. A panel may have a threshold of 175 volts rms. Above this threshold, the luminance increases logarithmically, an order of magnitude increase per 5 volts. As a percent of threshold, 5 volts is less than 3%. In other words, the expected luminance can change by a factor of ten if the threshold nonuniformity is 3%. It is difficult to hold such precision on a large part of this kind. This problem would also be ameliorated by a TFT-TFEL design. The objective of every flat panel display has been to match CRT performance. Since TFT-LCDs are in substantial production, investment in solving these problems in EL have waned. The widely used TFT-LCD panel uses a photoluminescent back light that is highly efficient, but only about 5% of that lamp luminance reaches the eye of the user because of characteristics of the LCD/color filter light valve.
What is therefore needed is a flat panel display that has neither the intractable mechanical problems with field emitter cathodoluminescent displays nor the problems that continue to plague electroluminescent displays. Further needed is a flat panel display having the relative efficiency of displays that stimulate phosphor. Further needed is a method of manufacturing/fabrication of cathodoluminescent/electroluminescent devices for use in flat panel displays that operate a lower voltage and lower frequency than conventional devices.
The invention is a flat panel display device having the phosphor stimulation capabilities of an FED. The flat panel device has the simplicity of manufacture of a TFEL display. A phosphor such a ZnS:Mn can act as both an EL phosphor and as a cathodoluminescent phosphor. The phosphor is deposited on a porous silicon underlayer that contains a labyrinth of fissures, voids, hillocks, and microscopically rough surfaces. At the phosphor-porous silicon interface, the labyrinthine surface possesses hundreds to thousands of electric field line compression points that can be characterized by an average field enhancement. When this underlayer is the cathode, high energy electrons are injected into the phosphor producing substantial light emission even at low applied fields. Additionally, the surrounding silicon is available to integrate drive circuitry and provide a TFT at each pixel, if needed.
The object of the invention is to provide a display device having the simplicity of manufacture found in TFEL display technology but without the characteristic high voltage and highly unstable threshold level.
Another object of this invention is to provide an electro-optical transducer embodied in a flat panel display that exhibits the efficiency of cathodoluminescence but does not require intractable mechanical structures to distally dispose an anodic plate from a needle-like cold cathode.